[RFC PATCH 7/7] lazy tlb: shoot lazies, a non-refcounting lazy tlb option

[Andy Lutomirski]

On Thu, Jul 9, 2020 at 6:57 PM Nicholas Piggin <npiggin at gmail.com> wrote:
>
> On big systems, the mm refcount can become highly contented when doing
> a lot of context switching with threaded applications (particularly
> switching between the idle thread and an application thread).
>
> Abandoning lazy tlb slows switching down quite a bit in the important
> user->idle->user cases, so so instead implement a non-refcounted scheme
> that causes __mmdrop() to IPI all CPUs in the mm_cpumask and shoot down
> any remaining lazy ones.
>
> On a 16-socket 192-core POWER8 system, a context switching benchmark
> with as many software threads as CPUs (so each switch will go in and
> out of idle), upstream can achieve a rate of about 1 million context
> switches per second. After this patch it goes up to 118 million.
>

I read the patch a couple of times, and I have a suggestion that could
be nonsense.  You are, effectively, using mm_cpumask() as a sort of
refcount.  You're saying "hey, this mm has no more references, but it
still has nonempty mm_cpumask(), so let's send an IPI and shoot down
those references too."  I'm wondering whether you actually need the
IPI.  What if, instead, you actually treated mm_cpumask as a refcount
for real?  Roughly, in __mmdrop(), you would only free the page tables
if mm_cpumask() is empty.  And, in the code that removes a CPU from
mm_cpumask(), you would check if mm_users == 0 and, if so, check if
you just removed the last bit from mm_cpumask and potentially free the
mm.

Getting the locking right here could be a bit tricky -- you need to
avoid two CPUs simultaneously exiting lazy TLB and thinking they
should free the mm, and you also need to avoid an mm with mm_users
hitting zero concurrently with the last remote CPU using it lazily
exiting lazy TLB.  Perhaps this could be resolved by having mm_count
== 1 mean "mm_cpumask() is might contain bits and, if so, it owns the
mm" and mm_count == 0 meaning "now it's dead" and using some careful
cmpxchg or dec_return to make sure that only one CPU frees it.

Or maybe you'd need a lock or RCU for this, but the idea would be to
only ever take the lock after mm_users goes to zero.

--Andy